In the case of sealed gas (CO2) laser oscillators, it is necessary to perform a gas exchange at regular intervals due to degradation of the laser medium gas and reduction in the gas purity due to inflow of external air leaking into the housing. In the conventional gas exchange for the sealed laser oscillator, evacuation is performed from the set pressure at the time of oscillation to 0.1 to 0.01 Torrs regardless of the frequency of gas exchanges (for example, see Patent Literature 1).